In an engine used in an automobile or the like, widely known is an exhaust turbocharger in which a turbine is rotated by energy of exhaust gas of the engine, and intake air is compressed by a centrifugal compressor directly coupled to the turbine via a rotation shaft and supplied to the engine, in order to improve the output of the engine.
A turbine rotor blade of a turbine used in the above exhaust turbocharger has a risk that a flow strain occurs in the exhaust gas flow flowing through a turbine housing due to the surrounding structure of the turbine rotor blade, and the flow strain becomes an excitation source which causes resonance in the turbine rotor blade and generates high-cycle fatigue.
For instance, as illustrated in FIG. 8, the flow velocity in the casing for housing a turbine wheel TW becomes lower as the flow approaches the wall surface. In the vicinity of a protruding portion 012 where a terminating end and a starting end of a scroll part of a turbine casing 010 meet, the flow velocity of the exhaust gas decreases, which causes a flow strain E of the exhaust gas flow. The flow strain E is likely to become an excitation source. In view of this, it is necessary to adjust the natural frequency of the turbine rotor blade to be outside the operation range.
Especially in a variable-geometry turbocharger (VG turbocharger), as illustrated in FIG. 9, a nozzle wake (nozzle interaction swirl) F generated at the downstream end of a stator blade nozzle 014 at the upstream side of the turbine wheel TW becomes an excitation source, and thus there is a risk of high-cycle fatigue.
In this case, the excitation frequency is the number of nozzles×the rotation speed, and the resonance is likely to occur in a high-order mode which is a relatively high frequency, or especially in a secondary mode.
As described above, in a variable-geometry turbocharger, resonance is likely to occur in a high-order mode which is a relatively high frequency, or in a secondary mode in particular. Thus, if the resonance of the secondary mode cannot be avoided in an operation range with a high rotation speed, the opening degree of the nozzle of the stator blade is limited to restrict a vibration force applied to the rotor blade, in order to avoid high-cycle fatigue. In this case, there has been a problem of not adequately taking the advantage of the characteristic of the VG turbocharger that the flow rate is freely adjustable within the operation range.
As to the resonance mode of the turbine rotor blade, illustrated in FIG. 10A is an example of the primary mode. A large amplitude part S1 is present at the distal end portion of the trailing edge of the turbine rotor blade 016 in the blade height direction. Further, illustrated in FIG. 10B is an example of the secondary mode. Large amplitude parts S2, S3 are present at respective distal end portions of the leading edge and the trailing edge of the turbine rotor blade 106 in the blade height direction. There is a node S4 between the strong amplitude parts S2, S3.
As to the variable-geometry turbine with variable nozzles, Patent Document 1 (JP2009-185686A) can be mentioned as a conventional technique for reducing a vibration force applied to the turbine rotor blade and restricting resonance of a turbine blade.
Patent Document 1 discloses a variable-geometry turbine including a turbine wheel having turbine blades, and nozzle vanes disposed around the turbine wheel. The nozzle vanes are rotatably supported by vane shafts. The vane angle of the nozzle vanes is adjusted to adjust the opening area of the nozzles. The vanes shafts of the nozzle vanes are arranged at a predetermined pitch along a circle, and the center of the circle is eccentric from the rotational center of the turbine wheel in the radial direction.